Introduction: Orthostatic intolerance commonly occurs in astronauts
returning to earth. Head-down tilt bedrest (HDTBR), which models
cardiovascular adaptation to weightlessness, decreases orthostatic
tolerance by 34–60% without any countermeasure. We hypothesized
that daily six hours lower-body-negative-pressure (LBNP,
- 25 mmHg) ameliorates orthostatic tolerance, plasma volume, and
cardiovascular deconditioning during HDTBR.
Methods: We submitted 23 healthy persons (12 women,
34.5 ± 9 years, 23.9 ± 2.8 kg/m²) to 30 days of strict HDTBR
(SANS-CM study). Subjects were assigned to 6 h upright seating
(positive control, n = 11) or - 25 mmHg LBNP (n = 12) per day.
We measured left ventricular outflow tract diameter (LVOT) and
LVOT-stroke volume by pulsed wave doppler echocardiography
during 15 min of 80 head-up tilt testing (HUT) with incremental
LBNP until presyncope before and after HDTBR. We determined
plasma volume with CO-rebreathing two days before and at HDTBR
day 27.
Results: With HDTBR, orthostatic tolerance decreased 289 ± 89 s
(- 23%) in the seated and 284 ± 95 s (- 22%) in the LBNP group
(p\ 0.001 vs. baseline, p = 0.968 between groups). Plasma volume
decreased 569 ± 114 ml in the seated and 604 ± 104 ml in the
LBNP group (p\0.001 vs. baseline, p = 0.813 between groups).
While supine stroke volume decreased 8 ± 1 ml in the seated and
9 ± 4 ml in the LBNP group (p\0.001 vs. baseline, p = 0.874
between groups), supine cardiac output did not change in either
group. Both groups showed similar reductions in upright stroke
volume following HDTBR, however, stroke volume at presyncope
did not change with HDTBR.
Conclusions: Six hours daily moderate intensity LBNP or seating did
not fully attenuate orthostatic intolerance, plasma volume loss, or
cardiovascular deconditioning during 30 days HDTBR. However,
both interventions better maintained orthostatic tolerance compared
with previous 30–60 days HDTBR studies without countermeasures